by Deborah Conn

The microprocessor leg has been with us for nearly a decade. Now it’s time to give feet and ankles a turn.

The Proprio Foot®, developed by Ossur and introduced in 2006, was the first commercial entry into the field of microprocessor-controlled foot/ankle systems. Two other devices—the iPed® and the Power Foot®—are closing in on release dates, and more are likely to follow.

It’s an exciting—and unpredictable—time for this technology. Chris Johnson, director of engineering for College Park Industries (CPI), in Fraser, Mich., puts it like this: “Whenever we see a new branch of technology take off from an existing branch, it generates a period of diverse solutions. After a while, things settle into more similar patterns. The evolutionary branch of conventional prosthetic feet is very well developed. Now we’re branching off into computer-controlled feet and it will be a wild period, as the various manufacturers will seek to meet the challenges of computer-controlled feet in diverse ways.”

The Proprio Foot
The name of Ossur’s Proprio Foot refers to proprioception, the sense of where a limb is in space, which enables the human body to react to its environment.

According to Ian Fothergill, Ossur Academy manager for Ossur Americas, the device’s onboard sensors are able to track the motion of the Proprio Foot, allowing it to adjust to stairs and uneven terrain.

The sensors use accelerometry technology, which discerns the direction and speed of the foot’s movement. For each stride, the device “traces” the foot as it moves through space, identifying when the user is walking on a flat or sloped surface, moving up or down stairs, getting up from a chair, or simply sitting.

“We get a clear analysis of the step-by-step movement of the amputee,” he says. “The sensors feed that information directly to a microprocessor that sends commands to the actuator, or motor.”

As a result, when the foot is leaving the ground, the computer selects the position for the next step. It automatically lifts the toe during swing phase to allow sufficient clearance and reduce the need for a user to hike up his or her hip.

Fothergill says, “As users walk up or down slopes or stairs, the foot adapts to optimize the ankle position, so users experience the appropriate heel-toe gait. This reduces the need to compensate and allows them to maintain an upright, balanced posture. Restoring more normal biomechanics on uneven terrain affects users’ stability, but it also reduces physical effort and strain on the joints and ligaments and improves balance and traction. The desirable result is that elderly amputees who did not leave their homes because of the risk of injury may now be able to do so confidently and safely.”

With its microprocessor and battery, the Proprio Foot is slightly heavier than a conventional prosthetic foot, but even so, weighs just about 2.2 pounds. “There are conventional feet on the market close to that,” says Fothergill, “so the weight is not a drawback.” The battery is attached as close to the knee as possible to reduce the user’s sense of its weight.

According to Fothergill, Ossur has applied for an L code for the Proprio Foot to describe the microprocessor control feature of the Proprio Foot. He says, “Since no other existing code adequately describes this feature, we are hoping that a new code is awarded in 2009.”

While conventional wisdom holds that technologically advanced prostheses are typically intended for young, active amputees, Fothergill notes that the Proprio Foot was designed to overcome problems of the elderly diabetic population. “Its purpose is to help prevent falls,” he says. “The idea is to get people who have limitations out into the community again. Right now, we’re missing a lot of potential in helping middle-aged or older diabetics become active and avoid further complications of their disease.”

The iPed
The iPed also uses sensors, a microprocessor and actuators to mimic the function of an anatomical foot and ankle and adjust to changes in terrain. The device was developed by Martin Bionics LLC, a research and development company acquired in March 2008 by OrthoCare Innovations, based in Washington, D.C. Jay Martin, now director of the Advanced Systems Group for OrthoCare, says that the iPed focuses on providing the full range of motion of an anatomical foot and ankle, as well as real-time control in the stance and swing phases of gait.

“Someone can walk across a golf course, pick up a bag, walk down a hill and start running, and the iPed can accommodate those changes in real time,” Martin says.

“It is capable of adapting to force, speed and terrain changes in gait, making ambulation much easier.”

In December 2006, Martin Bionics licensed the iPed to College Park Industries. The development is proceeding as a collaborative effort.

CPI’s Chris Johnson says, “Our foot and ankle system is targeting the full range of sagittal plane motion. This allows the foot to match the terrain as a human foot would. If you step off a curb with the heel of a conventional prosthetic foot, for example, it cannot conform such that the toe can reach street level, but the iPed can.

“The real-time response of the iPed is another distinguishing feature. The iPed is designed to react to the ground at each step. This is important, because the terrain can change at each step.”

Johnson, a below-knee amputee, personally tests each iPed prototype. “Every prototype allows us the opportunity to make the next one better,” he says.

The iPed is still under development, and Johnson is reluctant to estimate when it will be commercially available.

Martin says, “There are a number of challenges in designing new branches of technology. As large-range-of-motion feet are new to the industry, studies are needed to determine the full impact of these new capabilities for amputees.  There is a lot to learn about how these technologies can affect users, and it is exciting to share these findings with the industry.”  

He continues, “With the iPed, we’re developing a fully adaptive design, choosing to manage the energy the body is putting into the system more effectively than conventional approaches.  Conventional foot designs largely try to optimize the amount of energy return on each step––providing a ‘spring’ return characteristic. But other factors must be addressed as well, including the spring return’s timing, angle of spring return, rate and amount of angular change during stance, and resistance to angular change.”  

According to Martin, the iPed addresses each of these factors to make walking more efficient and safe. He notes, “Because the iPed is able to adaptively accommodate for force, speed, and terrain changes through the full anatomical range of motion, it has capabilities that no other foot offers.”

Like the Proprio Foot, the iPed is intended for amputees with a range of function. “We’re approaching the design as being suitable for low- to moderate- to high-level ambulators,” says Martin. “It’s well suited for daily ambulation by people who are out in the community.”

One challenge with all microprocessor-controlled prostheses, says CPI’s Johnson, is predictability. “With a conventional prosthetic foot, I know exactly what it will do at all times and in all types of terrain and activities,” he explains. “It may not have the range of motion of a human foot, but it is very predictable. All manufacturers will face the challenge of predictability with computer-controlled feet, as the state of the technology is not yet able to directly connect the human brain to the control system of the prosthesis.”

The Power Foot
As its name implies, the Power Foot is not just microprocessor-controlled, but powered. The device actually propels the foot as it pushes off the ground during the gait cycle.

The Power Foot was developed by Hugh Herr, Ph.D., director of the Biomechantronics Research Group at the MIT Media Laboratory, with partial funding from a Department of Veterans Affairs grant.

The device was licensed to iWalk, based in Cambridge, Mass., which is creating and testing prototypes.

Because conventional prostheses only provide a passive spring response during walking, amputees using them expend about 30 percent more energy to walk at the same speed as able-bodied individuals. As a result, most amputees walk at a slower pace. The Power Foot generates energy for walking by using multiple springs and a small battery-powered motor. The energy produced from the forward motion of the person wearing the prosthesis is stored in the power-assisted spring and then released as the foot pushes off. Additional mechanical energy adds momentum.

“This is the first truly powered foot-ankle system,” says Herr. “It gives the amputee more than the amputee gives it.”

The Power Foot also addresses other issues. According to Herr, 70 percent of amputees suffer from back problems, often caused by the unnatural gait that results from using a conventional prosthesis and by the impact of the prosthetic foot on the ground. The Power Foot reduces that impact.

Richard M. Greenwald, Ph.D., founder and CEO of iWalk, notes that, like the other microprocessor feet, the Power Foot uses sensors to determine when the foot is on the ground and its motion and direction. “The sensors detect and feed information to the processing unit to allow the foot to make adjustments in real time,” he says. “With the Power Foot, amputees will be able to walk faster, walk longer, and have better control and power going up and down stairs and ramps.”

Greenwald, too, resists announcing when the Power Foot will be commercially available. “We’re in the stage of transferring the device from development to product,” he explains. iWalk is testing prototypes, often on Herr, who is a bilateral below-knee amputee, and will be conducting limited clinical trials as part of a U.S. Army research grant.

Greenwald expects the Power Foot to appeal to more active users, at least initially. “But one exciting opportunity,” he notes, “is that the technology will be beneficial to those who are not as active. Because the Power Foot requires less energy, it may really help amputees to move around more and reduce problems they may have with diabetes or other conditions.”

What’s next?
As the field develops, research will both take on new challenges and continue to refine current technologies. Ossur’s Fothergill says, “The Proprio Foot has generated a lot of interest in the research community. The current challenge is identifying the key characteristics of a prosthesis that positively affect an amputee’s safety and mobility. Only when we understand this complex situation can we compare the effectiveness of Proprio Foot and any future devices.

“It is still early days, and published research is limited. However, over the course of the next two to three years, I expect the publication of papers looking specifically at these characteristics and how they affect the amputee population.”

MIT’s Hugh Herr believes neuromuscular control is the next frontier. He plans to embed small, wireless implants into his own leg muscles, so that when the muscle contracts, the electrical impulse will send information to the prosthesis. Another area of interest for Herr is the direct attachment of the prosthetic foot onto the bone of the residual limb, eliminating the need for uncomfortable sockets.

Until the industry can achieve true neural integration, Johnson anticipates further refinement of the sensors. “The challenge is to not add so many sensors that the device becomes too complicated, but to have enough so the device is intelligent enough to have an adequate level of predictability.”

Martin says he is addressing the challenge of designing effective control strategies. “The prosthetic system must function in symmetry with the anatomical system,” he says. “Allowing a computer-controlled prosthesis to ‘think, respond and react’ to the environment, much like the anatomical ankle does, provides a significant engineering challenge.

“And as other inputs, such as brain-to-prosthesis control strategies mature, prostheses will have even more capabilities.  This is an area that we are actively pursuing––developing the next-generation iPed’s control system to communicate not just with autonomous control, but also with the anatomical system.

“Just as mechanical feet have been around for a long time and are getting better, even the most technologically advanced feet will require much improvement,” says Jay Martin. “We still have a long way to go to fully replicate anatomical limbs."

Deborah Conn is a freelance writer based in Falls Church, Virginia.